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Everyone has been in a shower where you can tell when someone flushes a toilet, opens a kitchen tap, etc, as the temperature rapidly changes. There's also other places where the shower is virtually unaffected by anything else.

Is there a way of plumbing in lines (either using a certain size pipe, or certain order of branches) to minimize the sensitivity? Do pro plumbers ever pay attention to this, or do they just do whatever is the shortest run?

In my current house, the shower never changes temperature, despite use of dishwasher, laundry, toilets or other taps. There is a 3/4" line that runs one direction for cold, and another 3/4" line running the opposite direction for hot (the hot water tank is on the exact opposite side of the house from where the cold water comes in). On the cold side, there is the kitchen sink first, and then the bathroom (and I'm 99% sure based on positioning that the shower is first, followed by toilet, then sink). On the hot side, there is a 1/2" line that comes off first for the laundry and downstairs half-bath, and then the bathroom in the same order as before.

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    Are you sure that your current shower isn't thermostatically controlled? This will ensure that the temperature remains constant despite other users taking either hot or cold water.
    – ChrisF ♦
    Commented Aug 15, 2010 at 17:55
  • I had assumed that I didn't have one because it's only a single control, but upon closer inspection it is a Moen, which do make several single-control thermostatic controls which look somewhat like mine (mine is just likely at least a few years old). The previous owners kept good records, and re-did the bathroom a few years before I bought the place - I just looked through them, and it is in fact a pressure balanced valve.
    – gregmac
    Commented Aug 15, 2010 at 22:18
  • This is one of the pros of using a PEX system. The system has a hot and cold water manifold, with dedicated lines to each fixture so the pressure loss is more stable.
    – Tester101
    Commented Dec 17, 2010 at 17:09
  • A pressure-balanced mixer goes a long way toward moderating temperature extremes. Instead you'll notice changes in flow volume as the mixer cuts down the flow of (for example) hot to match a decrease in the flow of cold.
    – wallyk
    Commented Jan 19, 2018 at 3:23
  • Just make sure the supply lines are big enough to handle the demand. I'm living in a house that was completely renovated two years ago. We can flush the toilet, and the flow of water in the shower remains unchanged. Use the area of a circle formula (area = pi * r-squared).
    – Jim Fell
    Commented Oct 13, 2020 at 19:10

1 Answer 1

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One way to ensure that the shower temperature doesn't vary is to install a thermostatic mixer. While you might get reduced flow when someone else uses water in another part of the house the temperature will remain constant.

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  • Most thermostatic mixers have a delay before they correct the temperature.
    – Walker
    Commented May 17, 2016 at 17:16
  • @Walker You're going to get a delay with pretty much any shower. The water has to be heated somewhere even if you had an "instantaneous" heater immediately behind the shower head. Once the water has reached the right temperature it's not going to fluctuate.
    – ChrisF ♦
    Commented May 17, 2016 at 17:39
  • The issue with combi boilers (tankless boilers) is that the hot water flow rate often decreases more then the cold water flow rate when a tap is opened. In other homes both the cold and hot flow rate reduce, so the shower gets hotter, as the boiler takes some time to cool down. Most thermostatic mixers don't respond until after the tap is closed!
    – Walker
    Commented May 17, 2016 at 18:14

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